In order to avoid a situation where the wheels of the motor vehicle lock up when the brake is actuated due to an excessive brake pressure applied by the vehicle driver and, as a result, the motor vehicle loses its stability or steerability, vehicle brake systems are usually fitted with a brake slip regulation system (ABS), in which the brake slip is adjusted to an optimum brake slip.
In the case of brake slip regulation, the brake pressure is in each case automatically modulated, i.e. lowered, held constant and raised again, independently of the brake pedal force applied by the vehicle driver, at least in part of the pressure-medium-actuated brake system, if a risk of one or more vehicle wheels locking up is detected, until there is no longer a risk of locking up. In very general terms, therefore, brake slip regulating devices for motor vehicle brake systems have the task of ensuring directional stability and steerability of the vehicle combined with the shortest possible stopping and braking distances, especially when the roadway is slippery and the service brake system is fully applied.
On roadways with friction coefficients that are significantly different as between the right and the left (split coefficient), however, there may be a reduction in the directional or driving stability of the vehicle owing to the very large difference in the effective braking forces which then occurs at the right-hand and left-hand vehicle wheels. This severe asymmetry or lack of balance in the effective braking forces on the right-hand and left-hand side of the vehicle produces a braking yaw moment of greater or lesser magnitude in accordance with these asymmetrical forces, rotating the vehicle about the vertical axis thereof. In order to counteract this and to maintain directional or driving stability, i.e. to keep the vehicle on course, the vehicle driver can actuate the steering wheel by way of correction in order to produce a yaw moment acting counter to the braking yaw moment.
In the case of vehicle brake systems with protection against locking up, there is therefore generally a conflict of aims in such situations. On the one hand, the aim is to achieve braking and stopping distances which are as short as possible when braking but, on the other hand, it is also important to maintain directional or driving stability and steerability of the vehicle when braking.
In this context, manufacturers of brake-slip-regulated motor vehicle brake systems generally give higher priority to maintaining directional or driving stability and steerability of the vehicle than to achieving the shortest possible braking distances.
In order to maintain the directional and driving stability of the vehicle, the ABS control strategy is adapted in such driving situations. In this case, at least the wheels on one axle are subject to antilock control on the “select-low” principle, for example, i.e. they are controlled in dependence on the vehicle wheel currently operating with the lowest friction coefficient. This means that, in the operating situation described above, the brake of the wheel rotating on the higher friction coefficient is supplied only with the same, comparatively low brake pressure as the brake of the other wheel, that rotating on the lower friction coefficient, even though it could in fact be braked more strongly without locking up because of the higher friction coefficient prevailing at this wheel. In this case, therefore, braking forces of the same high magnitude or the same low magnitude are applied to both wheels, the result being that they do not contribute anything to the production of a braking yaw moment. Since the wheel rotating on the higher friction coefficient is braked less strongly than is possible, it has a correspondingly high potential to carry lateral forces, and this benefits the directional or driving stability of the vehicle. However, the good directional and driving stability is obtained at the expense of longer braking distances since, with this control principle, the vehicle wheels rotating on higher friction coefficients are braked less strongly than would be permitted per se by the adhesion prevailing there.
Inasmuch as the two front wheel brakes in a hydraulic vehicle antilock brake system with rear wheels protected from locking up by the select-low principle are protected individually from locking up by dedicated devices, it is customary to attenuate the effect of any yaw moment that builds up due to braking forces of different magnitude at the right-hand and left-hand front wheel by “yaw moment modification” superimposed on the individual antilock control for the two front wheels. The superimposed yaw moment modification ensures that the brake pressure at the front wheel rotating on the higher friction coefficient is built up more slowly than is possible per se in order to give the vehicle driver additional time to respond, i.e. to countersteer, by the resulting delayed buildup in the yaw moment. The superimposed yaw moment modification also contributes to a deterioration in the achievable braking or stopping distance.
According to DE 602 17 834 T2, there is an electrically assisted steering system which intervenes during a split-coefficient braking operation under brake slip regulation in order to keep the vehicle stable by automatic steer inputs. By these stabilization measures, the ABS behavior can be made more aggressive, with the brake pressure at the wheel with the higher friction coefficient being increased more quickly, i.e. at a higher rate, until the slip threshold is reached.